The present invention relates to integrated circuits, and more particularly to protecting a switching voltage regulator from overshoot conditions.
Switching voltage regulator power supplies are well known and commonly used to generate a regulated output voltage from a higher DC supply voltage that is relatively unstable and noisy. FIG. 1 is a schematic block diagram of a step-down voltage regulator 100, as known in the prior art. Power stage 102 includes MOS transistors 104 and 106 that are alternately switched on and off thereby to generate a square wave signal filtered by the LC network 108. The output voltage signal Vout of LC network 108 is the regulated output voltage of voltage regulator 100.
The level of the output voltage Vout is controlled by varying the duty cycle of the signals applied to MOS transistors 104 and 106. To maintain voltage regulation, the output voltage Vout is fed back to analog-to-digital (ADC) control loop 112 where it is digitized and applied as an input signal to digital compensator 114. Digital compensator 112 is adapted to regulate the output voltage Vout by varying the duty cycle of the signals generated by pulse-width modulated generator block 114.
Switching voltage regulators often include over-current protection circuitry adapted to protect the output voltage during overload conditions which may cause the output current to overshoot. To protect against current overshoots, the output current Iout is sensed and delivered to an over-current protection circuitry. FIG. 1 shows a simplified current sensor 120 coupled to current sensing ADC 118. Under overload or short circuit condition, output current Iout increases beyond a predefined maximum level. Current sensing ADC 118 is adapted to detect if this predefined current level is exceeded and in response causes the duty cycle of the signals applied to power stage 102 to vary so as to limit output current Iout.
In order to prevent destructive overload, output current Iout is required to be limited very quickly. Under severe overload, such as a short circuit, output current increases quickly and can reach destructive levels in several switching cycles. For example, a buck regulator designed to deliver 10A of output current may produce as high as 30A of current during a few switching cycles. In order to prevent such destructions, a fast current limit circuit is required.
Conventional circuits are adapted to sense the output current once per switching cycle. FIG. 2 is a schematic diagram of a number of blocks disposed in a conventional switching voltage regulator. These blocks include a power stage 202, a current ADC 204, an over-current protection block 206, a compensator 208, and a loop ADC 210. Loop ADC 210 is shown as sensing the output voltage Vout of the switching voltage regulator (hereinafter alternatively referred to as regulator). Current ADC 204 is shown as controlling over-current protection block 206 in response to the output current Iout sensed using power stage block 202. The output signals of both over current protection block 206 and Loop ADC 210 are applied to compensator 208.
In order to sense the output current, a sensor is used. The current may be sensed by measuring a voltage drop across a current sense transformer or a current sense resistor. Alternatively, the current can be sensed as a voltage drop across the MOS transistors disposed in the power stage or the inductor. Over-current protection block 206 is adapted to protect the MOS transistors (switches) disposed in power stage 202 by turning them off once the maximum allowable current is exceeded. It is desired to sample the output current level frequently in order to ensure quick reaction in response to an overload condition.
It is commonly desirable to provide multiple voltage supplies. To achieve this, a multi-channel regulator may be used. In a multi-channel regulator, each output current is sensed independently in order to protect the multitude of output stages. FIG. 3 is a block diagram of the current sensing feedback circuitry 300 of a multi-channel regulator (not shown) adapted to protect against current over-shoots. Feedback circuitry 300 is shown as including N-channels, namely channels, 3021, 3022 . . . 301N with each channel including a current ADC 304i coupled to an associated power stage block 306i, where i is an index ranging from 1 to N. As is seen from FIG. 3, the output current of each channel 302i is sensed separately and digitized by an associated digital-to-analog converter 304i. A multi-channel regulator with a current feedback circuit such as that shown in FIG. 3 is not scalable. Since each channel has a dedicated current ADC, increasing the number of channels would require a corresponding increase in the number of ADCs.
FIG. 4 is a block diagram of the current sensing feedback circuitry 400 of a multi-channel regulator (not shown) adapted to protect against current over-shoots. Feedback circuitry 400 is shown as including an over current protection block 408, a current ADC 404 and a multiplexer 402 adapted to couple one of the power stages 406i to ADC 404. The output current in each channel is separately sensed and sequentially applied to ADC 404 using multiplexer 402. In other words, multiplexer 402 sequentially supplies the output current sensed from one of the output channels to ADC 404.
Since a multiplexer often has a much smaller physical size than an ADC, feedback circuit 400 of FIG. 4 overcomes the size limitations of feedback circuit 300 of FIG. 3. However in feedback circuit 400, the output signal is sampled once per switching cycle and each channel is digitized once every N cycles, where N is the total number of channels. This causes an N-cycle delay for each channel and is thus undesirable. One technique to overcome this problem is to include a relatively fast ADC adapted to digitize analog current signal in a fraction of the switching cycle period in order to be able to digitize the output currents from all the channels in one switching cycle. Including such an ADC would add to the complexity of the design, and increase the die size as well as the current consumption.